The well known vibrator hitherto used for the conventional concrete placing work comprises primarily such an electric motor, so called "double headed motor", that have a motive shaft penetrating through the rotor to project front and rear of the motor and is adapted for the whole body including the motor to become a vibrating body by means of the eccentric weights attached to both ends of the projecting motive shaft. In such vibrator, since the number of rotations of the motor is ordinarily 3000 r.p.m. or so, it is to be speeded up to an extent of 9000 r.p.m. when in vibration by the use of a separately provided frequency converter(s), generally called "vibration amplifier". The motive shaft rotating at such high speed exerts inevitably turning impacts of the eccentric weights directly onto the bearing portions thereof resulting in decreasing the ultimate useful life of the bearing into only 2.about.3 months.